From the B4 tuning manual-
• Use 30° of caster (optional #9593 caster blocks) if you need
increased steering entering corners and less steering exiting corners. It
will also be more stable when accelerating through fast bumpy track
conditions.
• Use 25° of caster (#9580 caster blocks) if you need less
amount of steering entering corners than the 30° blocks, but more
steering in the middle and exiting corners.
• Use 20° of caster (optional #9592 caster blocks) if you need
even less steering entering corners and more steering exiting corners.
hope this helps.